Conventionally, the EA modulator used in an optical communication system is used in combination with a semiconductor laser device as an optical transmitter module, and generally has the configuration as follows. That is, in this configuration, the temperatures of the semiconductor laser device and the EA modulator are set to be constant by using a temperature control mechanism and continuous light which is outputted by injecting a driving current into the semiconductor laser device is modulated by applying an electrical signal to the EA modulator so as to output the signal light. At this time, an extinction ratio at the time of modulation (hereinafter, referred to as ACER (Alternating Current Extinction Ratio)) which is an intensity ratio of an ON level and an OFF level of the signal light is stabilized usually by keeping the temperature of the EA modulator constant.
The usual optical transmitter module is described in “Semiconductor Laser” by Ito Ryoichi and Nakamura Michiharu, Baifuukan, 1989, p. 277. Further, the usual EA modulator is described in “Electron Lett. vol 28”, pp. 1157 to 1158, 1992.
Meanwhile, in the recent optical transmitter modules to be applied to optical communication systems, not only the improvement of transmission properties such as high speed and large capacity but also the size reduction, electric power saving and low cost are required. For this reason, in recent years, optical modules which do not require the temperature control mechanism are demanded. Conventionally, in the optical transmitter modules using the EA modulators, in order to keep ACER stable regardless of ambient temperature, the temperature control mechanism which keeps the temperature of the EA modulators constant is indispensable. This will be described in detail below.
The ACER is determined by an extinction curve at the time when a DC voltage is applied to the EA modulator (hereinafter, referred to as DCER curve). When the voltage is applied to the EA modulator, a laser beam inputted into the EA modulator is absorbed by the EA modulator, and a light output which is attenuated by the inputted laser beam is outputted from the EA modulator. The amount of attenuation depends on the voltage applied to the EA modulator (hereinafter, referred to as VEA), and a relationship between the attenuation amount and VEA is expressed by a DCER curve. The ACER is proportional to a difference in DCER (hereinafter, referred to as ADCER) between a high level of an electrical signal (hereinafter, referred to as VOH (Voltage Of High)) and a low level thereof (hereinafter, referred to as VOL (Voltage Of Low)) to be applied to the EA modulator. That is, when the difference in DCER between VOH and VOL is large, the ACER also becomes large. Therefore, when the DCER curve, the VOH and the VOL do not change, the ACER is constant.
Further, the DCER curve changes in accordance with the change in temperature of the EA modulator and the semiconductor laser device. This is because an absorption end wavelength of the EA modulator (hereinafter, referred to as λEA) and an oscillation wavelength of the semiconductor laser device (λDFB) to be inputted have different shift amounts depending on temperature. The difference between λEA and λDFB is called a detune amount, and the detune amount changes in accordance with the temperature. When the temperature is high, the detune amount becomes small, and when the temperature is low, the detune amount becomes large. When the detune amount is large, even if a voltage applied is increased, the EA modulator cannot sufficiently absorb light. This is because when the detune amount becomes large, confinement of electrons and holes in a light absorption layer of the EA modulator at the time of applying a voltage becomes weak, namely, the absorption factor of the EA modulator decreases, and thus, the DCER becomes small in a deep voltage area. FIG. 6 illustrates the change of the DCER curve depending on the temperature.
In FIG. 6, the horizontal axis represents an applied voltage to EA modulator VEA, and the vertical axis represents the extinction ratio DCER at the time of applying the DC voltage. As the temperature becomes lower, the DCER in an area with large VEA becomes smaller, and thus, ΔDCER which is equivalent to that at the high temperature cannot be obtained. For this reason, in the optical transmitter module using the EA modulator, the temperature control mechanism, which keeps the temperature of the EA modulator and the semiconductor laser device constant, is indispensable in order to keep the DCER curve constant.
Generally, the EA modulator is used in combination with the semiconductor laser device. However, when the temperature control mechanism is not necessary, a light output from the semiconductor laser device is lowered at the time of the high-temperature operation, and thus, the light output from the EA modulator is decreased.